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MONDAY, June 25, 2012 — Blood tests for vitamin D deficiency are some of the most frequently ordered tests at doctors' offices.

But two new tests approved by the Food and Drug Adminstration are often inaccurate, according to a study by scientists at the Loyola University Chicago Stritch School of Medicine, which was presented at ENDO 2012, an annual endocrine medicine meeting in Houston.

"There has been an exponential increase in the number of vitamin D tests ordered for patients," Earle W. Holmes, the study's lead researcher, said in a statement. "But our study of two newly approved tests showed they had pretty poor performance."

People commonly get vitamin D from exposure to sunlight as well as foods such as fish, eggs, and fortified milk. The vitamin aids with the absorption of calcium, which builds bone density and strength. Recent research has also shown it helps prevent osteoporosis, high blood pressure, cancer, heart disease and diabetes.

Holmes' research group studied the Abbott Architect and Siemans Centaur2 tests. The researchers used them to test 163 randomly selected blood samples. The tests were at least 25 percent off in 40 percent of the Abbott Architect tests and 48 percent of the Siemans Centaur2 specimens. The tests tended to overestimate vitamin D deficiency.

The report does not state how many hospitals around the country use these two tests.

Previous Vitamin D Tests Have Been Flawed

In 2009, one of the country's largest medical laboratories admitted to flawed vitamin D testing. The incident involving clinical lab services company Quest Diagnostics represented “the largest patient test recall I’m aware of in my 20 years in the business,” Robert L. Michel, editor of The Dark Report, a newsletter for pathologists that first reported on Quest’s action, told The New York Times.

The Times reported that the incident raised questions about vitamin D testing, which had surged because in recent years because of research highlighting its connection to serious health conditions.

While Quests' tests tended to overstate the amount of vitamin D in patients' blood, underestimating vitamin D can lead to over-treatment for deficiencies in the nutrient, Holmes said.

In rare instances of toxic vitamin D overdose, which causes excess calcium buildup in the urine and blood. Symptoms include nausea, vomiting and abdominal cramps. It is unknown how much vitamin D constitute an overdose, but several published cases of toxicity involve intake of more than 40,000 international units (IU), much higher than the usual dosages recommended for many vitamin D-deficient patients.

A 2010 Institute of Medicine report recommends daily intake of 600 IU of vitamin D for infants through adults age 70 and 800 IU after age 71.

Some studies suggest that too much of the nutrient could actually cause kidney and tissue damage.